This invention relates to the field of protecting electrical circuits against potentially damaging effects of electrical transients and, more particularly, to methods and apparatus for reliably protecting AC-powered user equipment from both over-voltage and under-voltage conditions.
Many types of electrical equipment are susceptible to malfunctions or damage due to transient impulses or line voltage conditions which exceed an acceptable voltage range or window. For example, computers and TV satellite receiver decoders are very sensitive to voltage transients. AC powered motors are subject to damage from overheating when driven by AC voltages greater than or less than an intended AC voltage operating range.
Since voltage transients may be caused by such things as lightning strikes, inductive load switching and physical shock to power lines, utility companies which supply electrical power have no practical ways of preventing such occurrences. The problems are particularly acute in rural areas.
Various circuits are known that disconnect electrical power from a load when the power output is outside acceptable tolerances. For example, a crowbar circuit shunts an AC power line to ground when an over-voltage condition occurs. The shunted AC power line causes either a fuse to blow or a circuit breaker to trip thereby disconnecting the AC line from the load. After the load is isolated from the AC line, a user must locate, identify and replace or reset the fuse or circuit breaker, respectively, in order to restore AC power to the user equipment. These actions are inconvenient and extend the equipment downtime.
Some over-voltage protection circuits automatically reset when the power supply voltage falls back within an acceptable operating tolerance. These circuits however, subject user equipment to erratic on-again, off-again operation when the AC power line voltage is irregular or marginal. This intermittent operation itself can lead to potentially damaging transients, particularly if the user equipment load is inductive. For example, intermittent operation of electric motor starter windings causes them to burn out.
Actuating a relay to disconnect the AC line from electrical equipment also creates transients within the over-voltage monitoring circuit. For example, an over-voltage monitoring circuit typically requires a small amount power to operate during normal operation. When the AC line voltage is above the maximum allowable voltage level, the monitoring circuit must actuate a high current relay to disconnect power to the electrical equipment. Actuation of the relay produces electrical noise by abruptly drawing a large current in relation to the quiescent current necessary to maintain normal operation of the monitoring circuit.
Typical over-voltage protection circuits use analog components to monitor the AC line voltage. Analog circuitry however, is more susceptible to electrical noise than digital circuitry and is harder to customize for different applications. In addition, digital components are less expensive, require less support circuitry, and are more reliable over a wider range of operating conditions.
Another drawback of known over-voltage protection circuitry is that a powerful power line transient may destroy a solid state over-voltage shunt device, effectively removing it from the circuit. The user equipment is thereby left completely unprotected against any subsequent transients.
Power disconnect circuits typically offer no protection against under-voltage conditions, i.e., where the AC line voltage falls below a predetermined minimum voltage, for example, 90 volts. This is frequently referred to as a "brown-out" condition. Motors connected to refrigerant compressors and similar loads can burn out under brown-out conditions. Computer disk drive damage can also be caused by brownouts. Most AC-powered user equipment in the U.S. is designed to operate within a line voltage range (i.e. 108-132 volts). Shunt circuits therefore do not adequately protect such user equipment from both under and over-voltage conditions.
Over-voltage protection circuits are designed to disconnect power from user equipment at a preset AC line voltage. The appropriate "trip voltage" depends on the type of electrical equipment. For example, power equipment may be designed to operate within an operating range of 120 VAC +/-15%, where as more sensitive computer equipment may require a tighter tolerance of 120 VAC +/-10%. For other types of electrical equipment it may be necessary to provide over-voltage protection but unnecessary to disconnect AC power in a under-voltage condition. Over-voltage protection circuits, particularly analog protection circuits, are not easily modified, and usually require complete redesigns to change trip voltage(s) or to accommodate a new load specification.
For electrical equipment that operates off three-phase power, phase monitoring is desirable in addition to voltage amplitude monitoring. For example, three-phase power equipment is designed to operate from a power supply having each phase offset 120 degrees from the remaining two phases. When any phase of the power supply is deviates from the 120 degrees offset, there is no longer an evenly distributed output of power to the electrical equipment. If the power supply is severely out of phase, for example, if two or more phases have a small or zero phase difference, the power supply can damage the three phase load. The phase and amplitude of a power supply must be monitored to effectively protect three-phase power equipment.
U.S. Pat. No. 4,999,730 discloses a voltage monitor and control circuit that uses analog circuitry to monitor the AC power line voltage signal. The circuit however, does not provide phase monitoring for polyphase power supply signals and also suffers the shortcomings of analog monitoring circuits as described above.
Accordingly, a need remains for a low cost, noise resistent, AC voltage and phase monitor and controller, that is useful or easily adaptable to protect a wide variety of AC-powered equipment.